1. Field of the Invention
The present invention generally relates to voice switches, and more particularly, to a digital voice switch wherein the threshold level of the switch is made variable to adapt to any changes in the noise level on a communications channel.
2. Description of the Prior Art
Voice switches are known in the art as devices which distinguish between vocal sounds and noise carried by a communications channel. Devices of this nature have a number of uses. For example, a communications system may include n voice input channels and m voice output channels, where n&gt;m. One such system is disclosed in the U.S. Pat. No. 3,927,268 Joseph A. Scuilli et al. to a common assignee with the present application. Voice switches are used to determine when there are vocal sounds on any of the n input channels. Only those channels carrying vocal sounds at any instant are connected to an output channel.
Prior art voice switches compare samples of a signal on a channel with a fixed threshold level. If the signal is above the threshold level, it is assumed the signal represents voice, otherwise it is assumed that the signal represents noise. Many vocal sounds result in a signal which has its greatest magnitude near the beginning and tapers off toward the end of the sound. In voice switches, the large magnitude front end portion of the sound is usually detected, and the remainder of the sound is passed through the voice switch by using a hangover time for switch connections to turn on. For example, when speech is detected, the voice switch is turned on to pass detected samples of the channel signal. Once turned on, the voice switch will remain on for a hangover period to insure passage of all samples of the sound. A hangover time of 150 milliseconds has been considered suitable.
Typically, the voice detector detects speech by detecting a given number of consecutive samples in excess of the threshold value. Detection of four samples in succession has been considered suitable.
Clipping of the front end of the sound would occur if the voice switch passed only those samples occurring during the 150 millisecond period following detection of four consecutive samples in excess of the threshold. To minimize front end clipping, all samples are delayed a fixed amount after detection before being applied to the output of the voice switch. Consequently, the voice switch detects speed prior to the time the beginning of speech arrives at the point where the voice switch controls passage of the samples to the output terminal. A delay time of 4 milliseconds has been found to be suitable.
A problem with the prior art voice switches is that they often clip much of the speech of low level talkers. The fixed threshold used in this type of switch is usually set at a level to reach a compromise between noise triggering and speech detection. However, the level of speech varies substantially from speaker to speaker. For a speaker whose level is near or below the level of the threshold, the action of this voice switch will tend to produce excessive speech clipping.